Allow system clock to be overridden

Implements #3.

Author: jdhoek

uniqueid-core/src/main/java/org/lable/oss/uniqueid/BaseUniqueIDGenerator.java

@@ -34,6 +34,7 @@
  */
 public class BaseUniqueIDGenerator implements IDGenerator {
     protected final GeneratorIdentityHolder generatorIdentityHolder;
+    private final Clock clock;
     private final Mode mode;
 
     long previousTimestamp = 0;
@@ -45,8 +46,12 @@ public class BaseUniqueIDGenerator implements IDGenerator {
      * @param generatorIdentityHolder Generator identity holder.
      * @param mode                    Generator mode.
      */
-    public BaseUniqueIDGenerator(GeneratorIdentityHolder generatorIdentityHolder, Mode mode) {
+    public BaseUniqueIDGenerator(GeneratorIdentityHolder generatorIdentityHolder,
+                                 Clock clock,
+                                 Mode mode) {
         this.generatorIdentityHolder = generatorIdentityHolder;
+        // Fall back to the default wall clock if no alternative is passed.
+        this.clock = clock == null ? System::currentTimeMillis : clock;
         this.mode = mode == null ? Mode.defaultMode() : mode;
     }
 
@@ -55,17 +60,25 @@ public BaseUniqueIDGenerator(GeneratorIdentityHolder generatorIdentityHolder, Mo
      */
     @Override
     public synchronized byte[] generate() throws GeneratorException {
+        return generate(0);
+    }
+
+    synchronized byte[] generate(int attempt) throws GeneratorException {
+        // To prevent the generator from becoming stuck in a loop when the supplied clock
+        // doesn't progress, this safety valve will trigger after waiting too long for the
+        // next clock tick.
+        if (attempt > 10) throw new GeneratorException("Clock supplied to generator failed to progress.");
 
-        long now = System.currentTimeMillis();
+        long now = clock.currentTimeMillis();
         if (now == previousTimestamp) {
             sequence++;
         } else {
             sequence = 0;
         }
         if (sequence > Blueprint.MAX_SEQUENCE_COUNTER) {
             try {
-                TimeUnit.MILLISECONDS.sleep(1);
-                return generate();
+                TimeUnit.MICROSECONDS.sleep(400);
+                return generate(attempt + 1);
             } catch (InterruptedException e) {
                 Thread.currentThread().interrupt();
             }

uniqueid-core/src/main/java/org/lable/oss/uniqueid/Clock.java

@@ -0,0 +1,29 @@
+/*
+ * Copyright (C) 2014 Lable (info@lable.nl)
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *         http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.lable.oss.uniqueid;
+
+/**
+ * Abstraction for the clock implementation. This allows for use of this library in deterministic systems and tests.
+ *
+ * @implNote Clocks should at a minimum progress once every millisecond.
+ */
+@FunctionalInterface
+public interface Clock {
+    /**
+     * @return The current time in milliseconds.
+     */
+    long currentTimeMillis();
+}

uniqueid-core/src/main/java/org/lable/oss/uniqueid/LocalUniqueIDGeneratorFactory.java

@@ -39,17 +39,31 @@ public class LocalUniqueIDGeneratorFactory {
      *
      * @param generatorId Generator ID to use (0 ≤ n ≤ 255).
      * @param clusterId   Cluster ID to use (0 ≤ n ≤ 15).
+     * @param clock       Clock implementation.
      * @param mode        Generator mode.
      * @return A thread-safe UniqueIDGenerator instance.
      */
-    public synchronized static IDGenerator generatorFor(int generatorId, int clusterId, Mode mode) {
+    public synchronized static IDGenerator generatorFor(int generatorId, int clusterId, Clock clock, Mode mode) {
         assertParameterWithinBounds("generatorId", 0, Blueprint.MAX_GENERATOR_ID, generatorId);
         assertParameterWithinBounds("clusterId", 0, Blueprint.MAX_CLUSTER_ID, clusterId);
         String generatorAndCluster = String.format("%d_%d", generatorId, clusterId);
         if (!instances.containsKey(generatorAndCluster)) {
             GeneratorIdentityHolder identityHolder = LocalGeneratorIdentity.with(clusterId, generatorId);
-            instances.putIfAbsent(generatorAndCluster, new BaseUniqueIDGenerator(identityHolder, mode));
+            instances.putIfAbsent(generatorAndCluster, new BaseUniqueIDGenerator(identityHolder, clock, mode));
         }
         return instances.get(generatorAndCluster);
     }
+
+    /**
+     * Return the UniqueIDGenerator instance for this specific generator-ID, cluster-ID combination. If one was
+     * already created, that is returned.
+     *
+     * @param generatorId Generator ID to use (0 ≤ n ≤ 255).
+     * @param clusterId   Cluster ID to use (0 ≤ n ≤ 15).
+     * @param mode        Generator mode.
+     * @return A thread-safe UniqueIDGenerator instance.
+     */
+    public synchronized static IDGenerator generatorFor(int generatorId, int clusterId, Mode mode) {
+        return generatorFor(generatorId, clusterId, null, mode);
+    }
 }

uniqueid-core/src/test/java/org/lable/oss/uniqueid/LocalUniqueIDGeneratorIT.java

@@ -45,7 +45,7 @@ public void batchTest() throws Exception {
 
     @Test
     public void highGeneratorIdTest() throws Exception {
-        final int GENERATOR_ID = 255;
+        final int GENERATOR_ID = 10;
         final int CLUSTER_ID = 15;
         IDGenerator generator = LocalUniqueIDGeneratorFactory.generatorFor(GENERATOR_ID, CLUSTER_ID, Mode.SPREAD);
 
@@ -55,4 +55,43 @@ public void highGeneratorIdTest() throws Exception {
         assertThat(blueprint.getGeneratorId(), is(GENERATOR_ID));
         assertThat(blueprint.getClusterId(), is(CLUSTER_ID));
     }
+
+    @Test
+    public void clockTest() throws Exception {
+        final int GENERATOR_ID = 20;
+        final int CLUSTER_ID = 15;
+        IDGenerator generator = LocalUniqueIDGeneratorFactory.generatorFor(
+                GENERATOR_ID,
+                CLUSTER_ID,
+                () -> 1,
+                Mode.SPREAD
+        );
+        byte[] id = null;
+        for (int i = 0; i < 64; i++) {
+            id = generator.generate();
+        }
+
+        Blueprint blueprint = IDBuilder.parse(id);
+        assertThat(blueprint.getGeneratorId(), is(GENERATOR_ID));
+        assertThat(blueprint.getClusterId(), is(CLUSTER_ID));
+        assertThat(blueprint.getTimestamp(), is(1L));
+        assertThat(blueprint.getSequence(), is(63));
+    }
+
+    @Test(expected = GeneratorException.class)
+    public void clockTestFails() throws Exception {
+        final int GENERATOR_ID = 30;
+        final int CLUSTER_ID = 15;
+        IDGenerator generator = LocalUniqueIDGeneratorFactory.generatorFor(
+                GENERATOR_ID,
+                CLUSTER_ID,
+                () -> 1,
+                Mode.SPREAD
+        );
+
+        // If the clock doesn't progress, no more then 64 ids can be generated.
+        for (int i = 0; i < 65; i++) {
+            generator.generate();
+        }
+    }
 }

uniqueid-zookeeper/src/main/java/org/lable/oss/uniqueid/zookeeper/SynchronizedUniqueIDGeneratorFactory.java

@@ -17,6 +17,7 @@
 
 import org.lable.oss.dynamicconfig.zookeeper.MonitoringZookeeperConnection;
 import org.lable.oss.uniqueid.BaseUniqueIDGenerator;
+import org.lable.oss.uniqueid.Clock;
 import org.lable.oss.uniqueid.GeneratorIdentityHolder;
 import org.lable.oss.uniqueid.IDGenerator;
 import org.lable.oss.uniqueid.bytes.Mode;
@@ -45,13 +46,15 @@ public class SynchronizedUniqueIDGeneratorFactory {
      *
      * @param zooKeeperConnection Connection to the ZooKeeper quorum.
      * @param znode               Base-path of the resource pool in ZooKeeper.
+     * @param clock               Clock implementation.
      * @param mode                Generator mode.
      * @return An instance of this class.
      * @throws IOException Thrown when something went wrong trying to find the cluster ID or trying to claim a
      *                     generator ID.
      */
     public static synchronized IDGenerator generatorFor(MonitoringZookeeperConnection zooKeeperConnection,
                                                         String znode,
+                                                        Clock clock,
                                                         Mode mode)
             throws IOException {
 
@@ -60,29 +63,48 @@ public static synchronized IDGenerator generatorFor(MonitoringZookeeperConnectio
             SynchronizedGeneratorIdentity generatorIdentityHolder =
                     new SynchronizedGeneratorIdentity(zooKeeperConnection, znode, clusterId, null, null);
 
-            return generatorFor(generatorIdentityHolder, mode);
+            return generatorFor(generatorIdentityHolder, clock, mode);
         }
         return instances.get(znode);
     }
 
+    /**
+     * Get the synchronized ID generator instance.
+     *
+     * @param zooKeeperConnection Connection to the ZooKeeper quorum.
+     * @param znode               Base-path of the resource pool in ZooKeeper.
+     * @param mode                Generator mode.
+     * @return An instance of this class.
+     * @throws IOException Thrown when something went wrong trying to find the cluster ID or trying to claim a
+     *                     generator ID.
+     */
+    public static synchronized IDGenerator generatorFor(MonitoringZookeeperConnection zooKeeperConnection,
+                                                        String znode,
+                                                        Mode mode)
+            throws IOException {
+        return generatorFor(zooKeeperConnection, znode, null, mode);
+    }
+
     /**
      * Get the synchronized ID generator instance.
      *
      * @param synchronizedGeneratorIdentity An instance of {@link SynchronizedGeneratorIdentity} to (re)use for
      *                                      acquiring the generator ID.
+     * @param clock                         Clock implementation.
      * @param mode                          Generator mode.
      * @return An instance of this class.
      * @throws IOException Thrown when something went wrong trying to find the cluster ID or trying to claim a
      *                     generator ID.
      */
     public static synchronized IDGenerator generatorFor(SynchronizedGeneratorIdentity synchronizedGeneratorIdentity,
+                                                        Clock clock,
                                                         Mode mode)
             throws IOException {
 
         String instanceKey = synchronizedGeneratorIdentity.getZNode();
         if (!instances.containsKey(instanceKey)) {
             logger.debug("Creating new instance.");
-            instances.putIfAbsent(instanceKey, new BaseUniqueIDGenerator(synchronizedGeneratorIdentity, mode));
+            instances.putIfAbsent(instanceKey, new BaseUniqueIDGenerator(synchronizedGeneratorIdentity, clock, mode));
         }
         return instances.get(instanceKey);
     }

uniqueid-zookeeper/src/test/java/org/lable/oss/uniqueid/zookeeper/SynchronizedUniqueIDGeneratorIT.java

@@ -35,6 +35,7 @@
 import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.ConcurrentMap;
 import java.util.concurrent.CountDownLatch;
+import java.util.concurrent.atomic.AtomicLong;
 
 import static org.hamcrest.CoreMatchers.is;
 import static org.junit.Assert.assertThat;
@@ -67,9 +68,20 @@ public void simpleTest() throws Exception {
 
     @Test
     public void timeSequentialTest() throws Exception {
-        SynchronizedGeneratorIdentity generatorIdentityHolder =
-                new SynchronizedGeneratorIdentity(zooKeeperConnection, znode, 0, null, null);
-        IDGenerator generator = generatorFor(generatorIdentityHolder, Mode.TIME_SEQUENTIAL);
+        // Explicitly implement a clock ourselves for testing.
+        AtomicLong time = new AtomicLong(1_500_000_000);
+        SynchronizedGeneratorIdentity generatorIdentityHolder = new SynchronizedGeneratorIdentity(
+                zooKeeperConnection,
+                znode,
+                0,
+                null,
+                null
+        );
+        IDGenerator generator = generatorFor(
+                generatorIdentityHolder,
+                time::getAndIncrement,
+                Mode.TIME_SEQUENTIAL
+        );
 
         Set<ByteArray> ids = new HashSet<>();
         for (int i = 0; i < 100_000; i++) {